Add required DLLs

Adding the DLLs required to actually run the game (Allegro and add-ons,
PhysFS, FreeType2) so that rebuilding them from scratch isn't required.
These were all built on Win7 using MinGW-provided G++ 4.8.1.
Make makefile more portable

Add new variables in the makefile for where to search for header files
and libraries so that the sam4 source code and build area can be
separate from the MinGW installation and not rely on default search
paths.